1. Field of the Invention
This invention relates to slip resistant nonwoven materials especially those useful in surgical or clean room environments. More particularly, the invention relates to nonwovens useful for forming disposable protective articles such as drapes and footwear for health care and clean room environments.
2. Description of Related Art
Nonwovens are generally understood to be random-oriented fibrous webs which are produced by a variety of fibrous web manufacturing technologies generally utilizing substantial levels of synthetic fibers, though natural fibers can also be incorporated.
In nonwoven manufacturing, web forming is often done xe2x80x9cdryxe2x80x9d. Wet laid nonwoven manufacturing however is also known as are air laid processes which often use natural wood pulp fibers.
Fibrous webs are manufactured by at least three general processes: the wet papermaking processes; the nonwovens web making processes; and the woven textile web making processes. These three overlap each other in cost and performance. Papermaking is the lowest cost, and the products produced are the less durable. Some low end nonwovens compete with high end papers. On the other hand, woven textiles are very durable and comparably very expensive. Some high end nonwovens compete with low end woven textiles. Nonwovens may be considered as the webs which lie in between paper webs and textile webs.
Nonwovens web forming technologies include carding, air laid, spunbond, meltblown, and wet laid. Examples of additional nonwovens technologies include DRC (double recrepe), co-forming, and film aperturing.
Carding:
Carding is the oldest nonwoven technology. The easiest way to describe to the carding process is to consider the process of brushing a dog""s hair. As the pet is brushed, some fibers are pulled out and gather in the bristles of the brush. Occasionally it is necessary to xe2x80x9creverse brushxe2x80x9d and remove the fibers from the brush. This is somewhat like carding. In the carding process, synthetic fibers of, typically approximately 40 mm in length, are brushed out of a bale of fibers (The actual term used in the industry is xe2x80x9ccombingxe2x80x9d.) The fibers which stick in the comb are then reverse brushed to pull them out of the comb and reorient them. At the same time they are reoriented, the fibers are laid down on a carrier screen to form a web. Orientation of these fibers is typically linear because the barbs on the rolls which do the combing are fixed in place. Thus virtually all of the fibers are lined up with the machine direction of the web. The cross direction orientation can be increased by additional processes such as randomizing and crosslapping. Because nonwoven carded webs have a high fiber orientation, it is generally necessary to do some type of additional bonding beyond that which occurs naturally with fiber entangling. Bonding may be done through chemical bonding, thermal bonding, or mechanical entanglement. Carded products utilize synthetic fibers, but also natural fibers like cotton and wool.
A limitation to the use of natural fibers in carding is that carding requires relatively long fibers to work well. Since carding occurs in air, it is also sometimes referred to as an air laid product or a dry laid product.
Air Laid:
Air laid web forming is more typically characterized by the fibers actually being deposited from an air stream onto a carrier fabric to form the nonwoven web. Webs formed by this process have high loft and high pore volume. Wood pulp fibers are the predominant fibers used in air laid manufacturing, and they are typically 2-3 mm in length. Longer, synthetic fibers can be added with lengths up to about 12 mm, but the most common usage is at about 4-6 mm due to machine handling considerations. The use of longer fibers is desirable in that they entangle better than wood pulp, and are generally stronger than wood pulp. Air laid webs are typically chemically bonded by spraying an emulsion polymer on both sides of the web. Bonding can also be achieved by the incorporation of synthetic fibers and the use of thermal bonding. Multi-bonding is used to describe air laid webs which have primary internal thermal bonding, and then a chemical topcoat to tie down loose fibers. Synthetic fibers are finding increased usage in these nonwovens.
Spunbond:
The spunbond process is characterized by the use of molten polymers, extruded through fine orifices to form essentially continuous fibers. Moving orifices and/or directed air streams cause the fibers to twirl around and overlap one another as they deposit on a moving carrier screen. The nonwoven web thus formed may be bonded by a degree of mechanical fiber entanglement; it may be bonded through final solidification of the molten fibers occurring after the web is formed and the fibers contact one another; it may be bonded through additional thermal treatments; or it may be bonded through chemical treatments. Nonwoven webs produced by the spunbond process are usually very strong, but have a high synthetic handfeel.
Meltblown:
The meltblown process is very similar to the spunbond process except for three major differences: 1) In the making of meltblown fibers, the air attenuation process used in fiber drawing causes the fibers to break with much shorter fiber lengths than in the spunbond process. 2) In the meltblown process, the design of the air flow used to draw the fibers out from the extrusion orifices also causes the fiber diameters to be much smaller than those found in the spunbond process, and 3) The fiber attenuation airflow used in the meltblown process occurs much closer to the extrusion orifice, and therefor it forms and cools the fibers before the extruded polymer has had a chance to molecularly orient itself. This creates fibers which are weaker than those found in the spunbond process. Because the meltblown process has shorter fiber lengths, there is more natural entanglement in the final nonwoven web, and some meltblown webs require no further bonding. Generally, however, thermal bonding via heated emboss rolls is the bonding method of preference.
Wetlaid:
The wetlaid nonwovens processes are basically just adaptations on regular papermaking machines. The manufacture of wet laid nonwovens utilizes three key variables compared to papermaking: 1) some synthetic fibers and/or longer other naturally occurring fibers (e.g. hemp) are included in the furnish; 2) selective chemical additives are used to properly disperse and suspend the synthetic fibers in the water slurry; and 3) the papermaking machine is redesigned and altered to allow better fiber handling and water drainage. The use of synthetic fibers in combination with wood pulp fibers in the wet laid process is limited by the degree to which the synthetic fibers can be kept suspended in water similar to the way this is done with wood pulp fiber. This suspendability, in turn, is affected by the density of the synthetic fibers, the surface wetting characteristics of the synthetic fibers, and the length of the synthetic fibers. Machine handling limitations in wet laid nonwovens generally rely on synthetic fiber lengths of about 8 mm or less. A high degree of bonding occurs through the normal entanglement of the fibers in the web forming process. Significant additional strength is sometimes generated by utilization of the bonding technology known as hydroentanglement.
DRC:
In the DRC (double re-crepe) process, a deliberately designed very weak sheet of paper is chemically bonded by design printing an emulsion polymer on one side of the sheet. The wet side of this sheet is immediately stuck to a large cylindrical dryer where the drying process begins. The drying process causes the sheet to stick to the dryer, and it is necessary to crepe the sheet in order to release it. The same sheet is then printed on the other side and dried and creped again. The resulting nonwoven product is far more durable than the original base sheet of paper, and the creping steps soften the hand feel and increase the absorbent characteristics of the sheet.
Co-Forming:
Co-forming is a process for adding wood pulp fibers to a molten polymeric fiber stream in a melt blown process. A key benefit of this process is the cost reduction associated with the wood pulp fibers, but the addition of the wood pulp fibers also brings about some different nonwoven web properties.
While both spunbond and meltblown webs are formed from fibrous extrusion of molten polymers, films may also be formed from molten polymers. Films which have been modified with perforations and the like to increase porosity are also considered nonwovens for purposes hereof.
There are three primary classes of fiber-to-fiber bonding found in nonwovens.
Mechanical fiber entanglement occurs when in the web forming process fibers come in contact with each other and become intertwined.
Mechanical entanglement may be enhanced through several different techniques. One such technique is called aperturing. In aperturing, small jets of water are used to blast through the web while it is still supported on a carrier wire. As the water jets penetrate the web, they carry adjacent fibers with them, thereby penetrating or entangling the fibers down into the web.
A variation on aperturing is called hydroentangling. In hydroentangling, smaller jets of water are used with high pressures. The same reorientation of fibers occurs as in aperturing, but to a much larger extent. Furthermore, the pressures in hydroentanglement are sufficient so that the jets of water actually have their direction reversed as they pass through the web and hit the carrier wire. This causes some of the water to come back up through the web, also carrying fibers with it again, only now in the reverse direction. Hydroentangled webs are fairly strong, and can approach or even exceed certain textiles. Hydroentangling is not only used as a primary bonding technology, but it is also used as a binderless lamination technology.
Another example of mechanical bonding is needle punching. Needle punching utilizes barbed needles which penetrate down through the web. This is an older process, and it used primarily with very thick webs, such as might be found in the carpet manufacturing industry and/or highloft batting manufacturing.
Another mechanical bonding is stitch bonding. Stitch bonding is literally sewing the fibers together, just as one would sew fabric together. Stitch bonding is not so much utilized as a lamination technique as it is as a primary bonding enhancement technique. Its use in nonwovens is not yet wide spread, but it is finding a home in the upscale products with high durability.
Chemical bonding involves bonding through the addition of adhesive-like chemicals. Chemical bonding is also referred to as xe2x80x9cresin bondingxe2x80x9d. The most common form of chemical bonding is achieved by running the web through a saturation bath of an emulsion polymer. In some cases complete saturation is not used, and the emulsion polymer is xe2x80x9cprintedxe2x80x9d on by use of a gravure roll. In other cases, the emulsion polymer is sprayed onto the surface of the web. And in still other cases, a chemical is applied which causes the fibers to become reactive and bond with each other.
Thermal bonding is accomplished in at least two different ways. In the simplest way, the synthetic fibers are subjected to an overall melt temperature which causes them to soften. As the softening occurs, the fibers stick together. As the fibers cool this stickiness becomes permanent. Typically a drying oven is used for this type of heat transfer, but heated smooth compaction rolls may also be used. In a more complicated way, a web is run though a heated emboss roll nip, and the male portion of the emboss roll imparts a pattern on the web at the same time it is melting and bonding the fibers within that pattern. The fibers can also be designed to consist of two components with different melt temperatures. These are called bicomponent fibers. As one part of the fiber melts, it is still relatively held extended in place by the other part of the fiber. But wherever a molten part of the fiber touches another fiber (molten or not) bonding will occur as the molten part of the fiber cools. Most typically this type of thermal bonding occurs via an overall exposure to heat (e.g. a drying over), but there are some cases where heated emboss rolls are also used.
Nonwoven blends or fabrics are formed of many synthetic and natural fibers. Nonwoven fabrics find a variety of uses in towels, wipes, disposable garments, and layering for sanitary napkins and diapers. Methods and apparatus for forming entangled nonwoven fabrics are further described in U.S. Pat. No. 3,485,706. Interfiber frictional contact provides strength to the fabric. In addition to that described above, increased strength is achievable by addition of various additives, binders or adhesives to increase interfiber attachment.
A variety of synthetic materials such as polyolefins have been used to form nonwoven fabrics. Thermoplastics finding use in nonwoven fabrics include polyethylene and polypropylene, and materials such as nylon, polyesters and other synthetics. Disposable absorbent fabrics have typically been comprised of a batt or absorbent portion which is covered with a nonwoven liner. The batt is often a cellulose fiber material or polyester fiberfill (U.S. Pat. No. 4,818,599 incorporated herein by reference) or fiberglass or the like.
Nonwoven blends or fabrics are produceable by meltblowing or spunbonding techniques. Such techniques can create a matrix of thermoplastic fibers engaging at least some of the matrix fiber spaced apart from each other. The individualized fibers are interconnected within the matrix by mechanical entanglement of the matrix fibers. The mechanical entanglement and interconnection of the matrix fibers forms the structure. Heat, adhesive or binder can be used to increase interconnections and interconnection strength of the matrix fibers.
Nonwoven webs can be formed by meltblowing and conforming, meltspinning techniques, collected as a tow and converted to staple fibers. Nonwovens can be prepared by carding or air forming.
Meltblown fibrous webs can be made from fibers formed by extruding molten thermoplastic material through fine die capillaries to form molten threads or filaments which are then attenuated using high velocity gas. The resulting fibers have diameters usually of less than 10 microns and can be collected on a forming surface in the form of a fibrous nonwoven batt with very small pore structures that can inhibit fluid flow as taught by U.S. Pat. No. 3,849,241.
If desired, particularly with a nonwoven comprising a laminated sheet with an absorbent batting middle layer, an absorbent composition from 5 to 90 percent of the matrix fiber can also optionally be included in the absorbent batting, for moisture retention. Such materials are taught in U.S. Pat. No. 4,757,825 incorporated herein by reference. U.S. Pat. No. 3,783,872 describes use of polyalkylene oxide hydrogels as powders or films. Cross-linked isocyanate-capped poly(oxyalkylene) glycols, polyurethanes and polyureas are described in U.S. Pat. No. 3,939,105 and U.S. Pat. No. 4,806,598.
Nonwovens and/or nonwoven laminates sandwiching natural or synthetic batting fibers, optionally incorporated superabsorbents, can be used to form useful cloth-like materials, wipes, napkins, paddings. If made in sheet-like thicknesses, these materials and laminates can be used to form cloth-like or garment materials.
Disposable garments can be fashioned from nonwovens and/or nonwoven laminates with natural or synthetic batting fibers. Disposable garments have gained acceptance particularly in surgical and clean room environments.
When protective garments are fashioned from cloth-like materials relying on nonwoven outer layers, one problem in some applications has been the low coefficient of friction of many of the synthetics such as polyolefin-based nonwovens.
When surgical footwear coverings are fashioned from such nonwovens, it has been found necessary or desirable to add frictional surfaces to the portion of the footwear contacting the floor to add traction and reduce the chance of slippage.